Today, a great scientist chooses the wrong hill to
die upon. The University of Houston's College of
Engineering presents this series about the machines
that make our civilization run, and the people
whose ingenuity created them.

On Sept. 5th, 1906, the
62-year-old physicist
Ludwig Boltzmann slipped a noose around his
neck and hanged himself. Boltzmann, more than
anyone, had shown us how to predict the behavior of
gases by describing moving molecules.

But he lived at poor peace with himself, and now he
despaired of being understood. He probably
committed that irreversible act because scientists
attacked his ideas about irreversibility.

I need to explain that: When molecules collide,
they bounce off one another's force fields with no
friction, no energy loss. If time ran backward, the
collision would reverse itself perfectly. But that
sets up an absurdity: Suppose you open an air tank
and air molecules begin rushing out. Then suppose
the motion of each molecule could somehow be
reversed. Wouldn't history itself run in reverse?
Wouldn't the molecules rush back into the tank?

That's as silly as it is logical. Time looks
directionless on the molecular level, where motion
is perfectly reversible. But nothing is so perfect
in our larger scale of sensory awareness. Here in
the visible world the past cannot be undone. Time's
arrow flies from past to future. Air never flows
back into the tank.

Boltzmann turned superb mathematics on the
question. He showed how rules of averaging won't
let such a reversal occur. In any large collection
of molecules, disorder continues increasing after
you reverse the motions. The gas must keep flowing
out.

The trouble is, his math didn't say why reversed
molecular motions won't reverse history. Classical
physicists, who hadn't bought his molecular
mechanisms, attacked Boltzmann. Soon after he died,
quantum mechanics took shape, and Heisenberg's
Uncertainty Principle said it isn't possible to
specify reversed motions accurately. In a quantum
universe, Boltzmann's math still makes perfect
sense, and the idea that you can reverse time is
nonsense.

Boltzmann was brilliant, but he had a history of
depression and mental illness. Now he couldn't
answer his critics, yet he knew he was right. He
said,

[theory] fills my thought and action ... no
sacrifice for it is too much for me ... [it is] the
content of my whole life.

Boltzmann's theory was, as we say, the
hill he chose to die upon. He despaired and committed
his terrible irreversible suicide just as Einstein
and the new breed of physicists were taking him very
seriously. Had he waited just a little longer, he
would have seen his genius triumph. His belief
faltered, but he'd put irreversible change in motion.
Time's arrow was in full flight. His ideas continued
moving outward, and, by now, they have touched the
whole of 20th-century physics.

I'm John Lienhard, at the University of Houston,
where we're interested in the way inventive minds
work.

Coveney, P., and Highfield, R., The Arrow of
Time: A Voyage Through Science to Solve Time's
Greatest Mystery, New York: Fawcett
Columbine, 1990.

The theoretical apparatus that Boltzmann put in
place was truly immense. He took James Clerk
Maxwell's ideas as a starting point and showed how
to describe macroscopic behavior from the behavior
of molecular movement. He built the bridges that
connect the kinetic theory of gases to continuum
thermodynamics.

He directed that his tombstone have carved upon it
his equation relating entropy to molecular
probability. That reflected justifiable pride in
his most important theoretical result.

The demonstration that I refer to in the episode is
his so-called H-theorem. It proves that increasing
entropy is inevitable in any spontaneous process in
an ensemble of molecules. In other words, the
second law of thermodynamics is derivable from
molecular considerations, with minimal
assumptions.